Molecular van der Waals symmetry affecting bulk properties of condensed phases: melting and boiling points

Qualitative analysis of the trends in melting temperatures T _m and boiling temperatures T _b of organic compounds and related substances supports a concept of van der Waals symmetry suggested earlier by the authors. Data for substituted methanes, ethanes, ethylenes, benzenes and cyclohexanes, reveal linear T _b versus M ^α patterns but usually non-uniform T _m versus M ^α patterns (where M is the molecular mass, α varies from 0.5 to 1). In the same set of isomers, T _m and T _b often change in different succession. To explain the observed trends, a local van der Waals field U that acts on a molecule in a solid or liquid phase, was represented by a sum of an averaged long-range field U ₀ from all molecules, and a short-range “contact” term W determined by a molecular shape. Variations of T _m and T _b among closely related substances correspond to different strength of perturbation of U ₀ ^(C) (in a crystal) and U ₀ ^(L) (in a liquid) by W. Higher T _b of isomers with medium-symmetry molecules (like 1,2-disubstituted benzenes and cyclohexanes) reflect the better fitting of their molecules to a local molecular environment in a liquid (that contains vacancies). Highly symmetrical quasispherical molecules (e.g., C₂Cl₆ or C₆F₁₂) produce a stabilised solid and destabilised liquid state, hence their molecular crystals easily sublime rather than melt at ambient pressure. Dedicated in memory of Professor Petr M. Zorky